1. Field of the Invention
The present invention relates generally to cooling water supply tanks having heat mixing prevention functions and passive high-pressure safety injection systems and methods using the cooling water supply tanks and, more particularly, to a cooling water supply tank having a heat mixing prevention function which can prevent internal cooling water circulation and heat mixing, which is caused by high-temperature and high-pressure steam injected into a core makeup tank and a hybrid safety injection tank when the core makeup tank and the hybrid safety injection tank are operated, thus restricting an increase in temperature from a free surface of makeup water to a lower portion in the tank, and maintaining a large density difference, whereby injection of cooling water into a nuclear reactor can be facilitated, and to a passive high-pressure safety injection system and method using the cooling water supply tank.
2. Description of the Related Art
As examples of conventional techniques pertaining to safety injection tank systems of emergency core cooling systems of nuclear reactors, AP600 type core makeup tanks (CMTs) were introduced in U.S. Pat. No. 5,268,943 shown in FIG. 1 and “Nuclear Engineering and Design” Vol. 186, p 279 to p 301, and a CARR (CP1300) type core makeup tank was introduced in NUREG-IA-0134.
In addition, as shown in FIG. 2, a hybrid safety injection tank (hybrid SIT) which can be operated both at a low pressure and at a high pressure was proposed in Korean Patent Registration No. 10-1071415 (Reg. date: Sep. 30, 2011), entitled “Passive high-pressure safety injection tank for SOB and LOCA”. This technique has a combined structure of a conventional low-pressure SIT (safety injection tank) and a conventional high-pressure CMT (core makeup tank). A pressure equalizing pipe is provided to equalize the pressure between the low-pressure SIT and a high-pressure compressor, and a motor drive valve or a pneumatic drive valve is provided on the pressure equalizing pipe and is used as needed.
Meanwhile, in the conventional technique, when high-temperature and high-pressure steam is injected into the core makeup tank or the hybrid safety injection tank, emergency core cooling water circulates in the core makeup tank or the hybrid safety injection tank. Thereby, the entirety of the emergency core cooling water is rapidly heated. Therefore, at an early stage, density between the core makeup tank or the hybrid safety injection tank and a nuclear reactor connected thereto becomes the same. As a result, it becomes impossible for the emergency core cooling water to be injected into the nuclear reactor by natural circulation. In other words, the drive force by which the emergency core cooling water is injected into the nuclear reactor is markedly reduced.
FIGS. 3A and 3B illustrate a problem of emergency core cooling water being heated early by internal circulation in the core makeup tank or the hybrid safety injection tank according to the conventional technique.
FIG. 3A shows vertical distribution of the temperature of cooling water in the core makeup tank or the hybrid safety injection tank when there is no circulation in the core makeup tank or the hybrid safety injection tank. If there is no internal circulation, only a free surface portion on which high-temperature steam makes contact with makeup water is heated by the high-temperature steam. Therefore, the temperature of the entire cooling water is merely slightly increased.
FIG. 3B shows vertical distribution of the temperature of cooling water in the core makeup tank or the hybrid safety injection tank when there is circulation in the core makeup tank or the hybrid safety injection tank. If there is internal circulation, not only the free surface portion on which high-temperature steam makes contact with makeup water but also the internal portion of the makeup water is heated by the internal circulation. Thereby, the temperature of the cooling water in the core makeup tank or the hybrid safety injection tank is relatively largely increased. As such, if the temperature of the internal portion of the cooling water is increased, a density difference (ρCMT−ρS) between the density (ρCMT) of the makeup water and the density (ρS) of the cooling water of the nuclear reactor is reduced. As a result, drive force by which cooling water is injection into the nuclear reactor is reduced.